1. Field of the Invention
The present invention relates generally to optical scanners and image forming apparatuses, and more particularly to an optical scanner and an image forming apparatus for exposing a scan object surface by scanning a light beam to form an image.
2. Description of the Related Art
In image forming apparatuses such as laser printers and digital copiers, a light beam emitted from a light source is modulated according to image information. The light beam passes through a polygon mirror and scan lenses, etc., and focuses on a photoconductor. The light beam scans the surface of the photoconductor in a predetermined direction (main scanning direction) to form a latent image (electrostatic latent image) on the photoconductor. Toner adheres to the latent image to visualize the image information.
FIG. 54 is a configuration of a general image forming apparatus such as a laser printer or a digital copier employing the electrophotographic method. Referring to FIG. 54, a laser light beam emitted from a semiconductor laser unit 1001 functioning as a light source unit is deflected and scanned by a rotating polygon mirror 1002. The deflected laser beam forms a light spot on a photoconductor 1004 functioning as a scan object medium via a scanning lens (fθ lens) 1003. The photoconductor 1004 is exposed so that an electrostatic latent image is formed thereon. A phase synchronizing circuit 1009 applies modulation signals generated by a clock generating circuit 1008 to a phase in synchronization with a photo-detector 1005. The photo-detector 1005 detects the laser light beam emitted from the semiconductor laser unit 1001 and deflected and scanned by the polygon mirror 1002. Specifically, the phase synchronizing circuit 1009 generates an image clock (pixel clock) whose phase is synchronized based on output signals from the photo-detector 1005 for each line, and the image clock is supplied to an image processing unit 1006 and then to a laser driving circuit 1007. Accordingly, the light emitting time lengths of the semiconductor laser unit 1001 are controlled through the laser driving circuit 1007, in accordance with image data generated by the image processing unit 1006 and the image clock whose phase is set for each line by the phase synchronizing circuit 1009. Thus, the electrostatic latent image on the photoconductor (scan object medium) 1004 can be controlled.
In recent years, requirements for faster printing speed have been addressed by enhancing the rotational speed of the polygon mirror and frequencies of clock signals used for modulating light from the light source. However, the printing speed cannot be enhanced sufficiently by these methods. In an attempt to achieve even faster printing speed, a multi-beam method has been devised, which employs plural light sources.
In an optical scanning method using multiple beams, more light flux segments can be scanned simultaneously by deflecting the light flux segments with a deflecting unit. Therefore, it is possible to reduce the rotational speed of a polygon motor functioning as a deflecting unit, and to reduce pixel clock frequencies. Thus, optical scanning and image forming can be performed at high speed and in a stable manner.
The light source employed in the multi-beam method is realized by, for example, a combination of laser chips that each emit a single beam, or an LD array or a vertical cavity surface emitting laser made by incorporating plural light emitting elements into a single laser chip.
These semiconductor lasers such as LD arrays employed in the multi-beam method are widely used as light sources in laser printers, etc., as they are extremely small, and can directly perform modulation at high speed by using a driving current. However, the relationship between the driving current and light output of semiconductor lasers changes with temperature, making it difficult to fix the light intensity of semiconductor lasers at a desired level. Particularly, in vertical cavity surface emitting lasers, light emitting elements are spaced apart by short intervals on a single chip; therefore, the temperature rapidly changes due to light emission and light quenching, and temperature crosstalk occurs significantly, causing variations in light quantity.
For example, in an optical scanner disclosed in Patent Document 1, plural light sources are disposed two-dimensionally, and plural light flux segments from the light sources are deflected by a deflecting unit to scan a medium. Intervals between light emitting points are minimized without causing heat crosstalk between the light emitting points.
Patent Document 2 discloses a method of controlling pixels of electrostatic latent images in an image forming apparatus that employs a vertical cavity surface emitting laser, by changing light emission intensity of each chip by pixel units, and controlling the light emitting time length.
Patent Document 3 discloses a method of avoiding heat crosstalk and increasing the density of recorded images in an image forming apparatus employing a vertical cavity surface emitting laser, by prescribing the arrangement of light sources.
However, in the conventional methods employed in optical scanners including plural light sources such as an LD array or vertical cavity surface emitting lasers, each light source is generally used for creating one pixel. Therefore, unevenness in light emitting levels between the light sources directly causes unevenness in image density. Particularly, in the conventional methods, there are no means for correcting uneven pixels in a sub scanning direction. Furthermore, when one light source becomes degraded, the light emitting level thereof decreases. The decrease in the light emitting level directly causes a decrease in image density. Moreover, in the conventional methods, there are no effective means for precisely correcting scanning lines bent due to irregularities in optical systems or mechanical components.    Patent Document 1: Japanese Laid-Open Patent Application No. 2001-272615    Patent Document 2: Japanese Laid-Open Patent Application No. 2003-72135    Patent Document 3: Japanese Laid-Open Patent Application No. 2001-350111